This invention relates to data communication systems and in particular to an analog data station terminal for synchronously interfacing an analog data communication device with a digital data communication network.
The public telephone network includes hard wire circuits consisting of two or more conductors connected between each telephone user and a central office, and switching equipment located at the central office which automatically routes calls from one customer to another. Because the public telephone network was originally designed to carry voice transmissions, the hard wire circuits are designed to provide a bandwidth of about 3 kilohertz. The advent of digital computers and data terminals brought about a need to have such devices communicate through the public telephone network. However, because digital devices must communicate with digital or binary encoded signals it is necessary to use a modem to provide such digital devices access to the analog based telephone network. A modem, which stands for modulator/demodulator, converts a digital signal to an analog audio signal which can be sent over the 3 kilohertz bandwidth limited circuits of the telephone network.
As computer technology and the need for faster communication have developed, the data transmission rates have steadily increased from about 300 bits per second to about 9600 bits per second and even higher, for example, as high as 56 kilobits per second. The telephone voice network however, is still designed to the basic 3 kilohertz bandwidth and reliable transmission of digital data over the voice network becomes increasingly difficult at the higher data rates. Voice-band telephone lines which have acceptable characteristics for voice signals are not suitable for high speed, modulated signals and so must be conditioned to prevent, or at least minimize, information loss. One method of line conditioning is equalization. Equalization is accomplished by manually adjusting active filters located at the telephone company's central office and at a customer's premises to compensate for amplitude losses and phase changes of signals between 100 and 3200 hertz. Equalization is a time consuming technique requiring special test equipment and highly skilled technicians. Accordingly, it can be a costly process.
Known devices for connecting a high speed analog modem to the telephone company's central office include a special terminal at the customer's premises, the four wire customer loop circuit, and a channel card located at the telephone company's central office. The central office channel card provides electronic line conditioning on signals from the customer premise to the central office. The terminal at the customer's premises provides electronic conditioning of signals from the central office to the customer. As indicated above, the earlier known systems require manual adjustment of the amount of compensation to provide the optimum bandpass for the data signals. Automatic equalization devices for making compensating adjustments at the customer premises have been developed which utilize a series of electronically generated tones selected in the bandpass. Although such automatic devices eliminate the need for manual compensation, the complexity, and hence the cost, of such automatic conditioning equipment is very high.
It would be highly desirable to eliminate the need for analog equalization by transporting the data signal in digital form from the customer's premises to the central office and vice versa. To achieve a digital signal that is compatible with the existing digital telephone network, it is necessary to sample and convert the analog signal generated by the modem in a manner which is synchronous with the telephone company multiplexing equipment at the central office.